1. Field of the Invention
The invention relates to a gas concentration detection device, and more particularly to a gas concentration detection device that detects the concentration of a specific component in exhaust gas discharged by an internal combustion engine.
2. Description of the Related Art
For instance, Japanese Patent Application Publication No. 2007-155605 (JP-A-2007-155605) describes an exhaust gas sensor system that detects the concentration of a specific component in exhaust gas of an internal combustion engine. This system has two sets of sensor cells. Each sensor cell has a diffusion resistance layer exposed to exhaust gas, and an electrode covered by the diffusion resistance layer. The sensor cells function as limiting-current type air-fuel (A/F) ratio sensors that produce an output in accordance with a gas component ratio on the electrode surface. On the surface of the diffusion resistance layer of one of the sensor cells (first cell), in particular, there is provided a catalyst layer for promoting decomposition and reaction of a hydrogen (H2) component.
In such a system, the H2 component contained in the exhaust gas is eliminated as the H2 reacts in the catalyst layer of one sensor cell (first cell). In the first cell, as a result, a sensor output from which the influence of the H2 component has been eliminated is detected. The other sensor cell (second cell), by contrast, has no catalyst layer, and hence the H2 component may penetrate into the diffusion resistance layer. The H2 component, having a high diffusion rate, reaches the electrode surface in a greater amount than that of oxidizing agents such as oxygen (O2) or nitrogen oxide (NOx). As a result, a sensor output denoting a richer A/F ratio than the actual A/F ratio is detected in the second cell, on account of the influence of the H2 component. H2 concentration is detected in the above conventional system on the basis of the difference between these sensor outputs.
In the above conventional system, however, the oxidation reaction of the H2 component in the catalyst layer may be restricted under conditions of a rich exhaust gas, i.e. under conditions in which the oxidizing agents in the exhaust gas are insufficient. The above conventional system, therefore, is limited to instances where the exhaust gas environment is lean and H2 concentration can be detected with good precision, and thus cannot readily cope with a wider variety of exhaust gas environments.